The marine, automotive, trucking, rail, aerospace, defense, recreation, chemical, infrastructure, and other industries look to composite materials to take advantage of their unique properties, especially being corrosion-free or corrosion-resistant and having a high strength-to-weight ratio. Composites are also resistant to fatigue and chemical attack. They offer high strength and stiffness potential in lightweight components. There is a need, however, to develop composite manufacturing processes that dramatically reduce the cost of composites, especially large structures, while retaining high strength and stiffness.
Open mold wet layup processing can make large composites using a liquid molding process with a small capital investment in single sided tooling, and often can use lower cost materials than resin-impregnated fibrous materials (prepregs). The quality and uniformity of the product, however, varies considerably. The best of these composites are still relatively low quality. The process also tends to be unfriendly and presents hazards to workers because of their risk of exposure to the solvents and resins.
High performance composites are currently made with prepreg. Woven or unidirectional tapes of the prepregs are placed on a forming mandrel (“laid up”) by hand or machine. Debulking (compaction) is often required between plies in a laminate to remove air before the laminates are vacuum bagged (i.e., enclosed in an inert atmosphere under vacuum to withdraw emitted volatiles released during cure of the resin) and consolidated (i.e., exposed to elevated temperature and pressure in a curing cycle) in autoclaves or presses to achieve high fiber volume components. The prepreg materials typically are expensive (especially those using high modulus carbon fiber). The raw prepreg materials have limited shelf lives because the resins that impregnate the fibers continue to react (“advance”) at ambient temperature. Advance of the resin adversely effects the properties of the resulting composite. Working with prepreg also often results in considerable material waste.
The autoclaves and presses used for consolidation to apply pressure to the laminated prepregs are expensive capital items that further increase the final, manufactured cost of the composite. Processing has to be centralized and performed in batches where the autoclave or press is installed. Loading and unloading the autoclave (a high temperature, pressurized oven) usually becomes the rate limiting step. The location of the autoclave dictates where the composites will be made, so the flexibility of the process is impaired. A dedicated workforce and facility are required, centered around the autoclave.
In some formulations, the resin in a prepreg is carried onto the fiber as a lacquer or varnish containing the monomer reactants that will produce the desired polymer in the composite (i.e., prepregs of the PMR-type). In other formulations, the resin is a relatively low molecular weight polymer that crosslinks during cure to form the desired polymer. The resin is held and used in its a state so that it remains a liquid, and can be impregnated onto the fiber or fabric. Reaction of the monomer reactants or crosslinking of the polymer (i.e., its advancing) prior to the intended cure cycle adversely impacts the quality of the composite.
Liquid molding techniques such as transfer molding, resin film infusion, resin transfer molding, and structural reaction injection molding (SRIM) typically require expensive matched metal dies and high tonnage presses or autoclaves. Parts produced with these processes are generally limited in size and geometry.
Infusion of dry preforms with wet resin with the use of vacuum (atmospheric pressure) as the only driving force is known. While there may be earlier examples, the Marco method (U.S. Pat. No. 2,495,640) was first used in the early 1940s. Palmer (U.S. Pat. No. 4,942,013) and Seemann (U.S. Pat. No. 4,902,215) are more recent examples. We are also aware of a number of other approaches covered in composite technology literature: RIRM, RIFT, and UV-VaRTM. Boeing's Double Bag Vacuum Infusion (DBVI) process, described in U.S. patent application Ser. No. 09/731,945, makes numerous claims regarding the control of the vacuum-assisted infusion with a resin distribution media, multiple porting, or channels. Seemann has been awarded other patents largely having to do with integration of a resin distribution matrix into a re-usable bag, such as U.S. Pat. Nos. 5,052,906; 5,316,462; 5,439,635; and 5,958,325.
The physics of the infusion process requires a pressure differential across the preform to drive the infusion of the resin into the preform. The traditional approaches infuse the resin at full atmospheric pressure, i.e., the reservoir from which the resin is being drawn is open to the atmosphere. During infusion as the preform fills with resin, the pressure inside the vacuum bag (i.e., the impervious outer sheet that contains the flow of resin during the infusion) in the filled volume approaches the pressure outside the bag, namely atmospheric pressure. Because vacuum-only resin infusion relies solely on the overpressure of the atmosphere to constrain the preform beneath the bag against the forming surface, this rise in pressure inside the bag reacts against the atmospheric pressure above. The remaining difference in pressure between that inside the bag and atmospheric pressure (i.e., the net compaction pressure) is all the pressure that is left to constrain the fiber preform on the forming surface. This pressure differential will vary depending upon a number of factors including the profile of the pressure gradient, hence the permeability of the materials being infused, and the timing sequence of clamping the inlet and exit lines. The finished thickness of a given preform is directly related to its finished fiber volume fraction. Achieving a high fiber volume fraction requires compaction of the preform. Compaction is achieved by pressing the preform against the forming surface. Proper constraint of the preform against the forming surface during and after infusion until the resin cures is critical to obtaining a high performance structure that results from its having a high fiber volume. If the net compaction pressure is insufficient (in traditional VaRTM, it can approach zero), the preform is free to float in the resin or to spring back from its compacted state, leading to reduced fiber volume fractions.
Seemann Composites, Inc. has produced a variety of composite structures for Boeing using the Seemann Composite Resin Infusion Molding Process (SCRIMP) from flat panels for making mechanical tests coupons (Boeing-Seattle, Fall 1999) to complex, demonstration wing structure (Boeing-LB 1998-2000) with the intention to use SCRIMP for making aerospace parts. A common problem experienced with these structures and panels has been lower than desired fiber volumes and concomitantly higher than desired finished thickness per ply for aerospace use. The preferred range for the carbon fiber volume fraction in aerospace composites is nominally at the higher end of that attainable, nominally 52-60% depending upon the preform being infused. The desired fiber volume is highly dependent upon the type of weave or other fiber architecture and the size and count of carbon tow for example. The laminates and structures Seemann Composites made for Boeing typically had a fiber volume fraction lower than the desired range. Control of the composite thickness through the inches per ply metric is important in order to control the resulting weight of the composite. In traditional resin infusion failure to optimize the thickness often means that each ply is thicker than necessary. Resin lacking fiber reinforcement has poor strength, so uncontrolled plies in a laminate can form a pattern of high strength areas sandwiched between lower strength areas. The overall laminate will have lower strength than a properly consolidated laminate having the optimal per ply thickness, and will generally require more plies to achieve the desired strength. More plies translates to more material and more labor, making already expensive parts even more expensive. It also translates to more weight, reducing overall performance of the aerospace system in which the composites are used.
As described in U.S. Pat. No. 4,902,215, Seemann induced preferential flow and pressure in the flow media above the fiber preform inside the vacuum bag to distribute the infusing resin in a network over the preform. The driving force is a pressure differential or head pressure created primarily by drawing down the pressure inside the bag using a vacuum pump. Atmospheric pressure on the resin pushes resin into the bag through an inlet tube. Resin entering the bag encounters the flow media used to channel the resin to the underlying fiber preform. Resin flows laterally through the flow media over the preform and, subsequently, downwardly into the preform. The preform normally has the lowest permeability to flow (i.e., the highest resistance to the flow of resin).
Some have proposed to obtain higher fiber volumes by adopting a process that could be simply described as fill or feed and bleed. Here the preform is infused using full atmospheric pressure to push the resin into the dry preform as is done with traditional resin infusion—the fill step. After the preform is fully infused the inlet line(s) are clamped and then the preform is exposed to full or nearly full vacuum at the exit(s) or at both the exit(s) and inlet(s)—the bleed step. The bleeding of the preform will result in higher fiber volumes. However, the fill and bleed process lacks control and is a timed process lest too much resin may be removed from the preform, either locally or throughout the preform. Moreover, the different permeabilities with the assemblage of different preform components, media, etc. complicates any ability to control the bleed and the resultant fiber volume. By Darcy's Law resin will flow from the infused preform more rapidly from areas of higher permeability. Practical structure commonly found in aerospace design would likely contain such differences in permeability in a given preform assemblage.
The resin infusion process of the present invention ensures that the fiber plies in the preform will remain compacted, that the preform is completely filled when the infusion is halted, and that optimum fiber volume fractions are achieved, thereby improving the traditional infusion processes.